Smith-Lemli-Opitz Syndrome (SLOS) is an autosomal recessive disorder caused by an inborn error of cholesterol biosynthesis. SLOS is estimated to have an incidence of 1:1,590 to 1:60,000 live births, making it the fifth commonest recessive disorder in Caucasians (Am. J. Med. Genetics, 2006; 140; p 2057-62). Clinical manifestations of SLOS are extremely variable. At the severe end of the spectrum SLOS is a lethal disorder, typical features of which include microcephaly, postaxial polydactaly, second and third toe syndactaly, cleft palate, genital anomalies, growth failure, and mental retardation. Mild SLOS cases combine minor physical stigmata with behavioural and learning disabilities. Intrauterine growth retardation is common and postnatal growth failure affects the majority of SLOS infants. At least 95% of SLOS patients present with some degree of mental retardation and learning disabilities. (Mol Genet Metab 2000; 71(1-2):163-74; Ann Hum Genet 2003; 67(Pt 3):269-80)
Biochemically, SLOS is caused by a deficiency in 3β-hydroxysterol Δ7-reductase. This is an NADPH-dependent ER enzyme that catalyses the reduction of the C7(8) double bond of the class II amphiphile 7-dehydrocholesterol (7-DHC) to yield cholesterol in the final step of cholesterol biosynthesis, via the Kandutsch-Russel pathway (FIG. 1) (Am J Hum Genet 1998; 63(1):55-62). This results in decreased cholesterol levels in all cells and tissues and an increase in sterol precursors. In particular, this results in an increase in the cholesterol precursor 7-DHC (Mol Genet Metab 2000; 71(1-2):163-74).
One potential therapy for SLOS patients is to increase dietary cholesterol to compensate for the endogenous block in cholesterol biosynthesis. However, in patients this therapy produces only limited clinical benefit. Increasing dietary cholesterol does not significantly alter the level of cholesterol or 7-DHC in the cerebrospinal fluid, which may explain why mental improvement in response to this therapy is very limited (Mol Genet Metab 2000; 71(1-2):154-62). For a more effective therapy it may be necessary to reduce 7-DHC storage in addition to increasing cholesterol levels (Mol Genet Metab 2000; 71(1-2):163-74). Current therapies combine increasing exogenously delivered cholesterol whilst attempting to lower 7-DHC levels by inhibiting de novo sterol synthesis using simvastatin (Mol Genet Metab 2005; 85(2):96-107). There is however a need for greater understanding of the effects of 7-DHC elevation within the cell, which may lead to the identification of additional targets for therapeutic intervention.
Cells obtain cholesterol either by de novo synthesis (FIG. 1) or exogenously by binding of LDL particles to LDL-receptors at the plasma membrane (PM) and subsequent endocytic uptake in clathrin coated pits (FIG. 1) (Mol Genet Metab 2002; 75(4):325-34). Free cholesterol is released from the LDL particle via the action of acid lipase in the late endosome. From here it is transported to the ER where it can be recycled to the PM or esterified. As current SLOS therapies focus on increasing the delivery of exogenous cholesterol, one recent study (Mol Genet Metab 2002; 75(4):325-34) has focussed on the effect of increasing intracellular concentrations of 7-DHC on LDL-derived cholesterol internalization, transport, and metabolism. Growth of SLOS patient fibroblasts in lipoprotein deficient serum (LPDS, which contains no LDL) lowers exogenously derived cholesterol. This results in up-regulation of the endogenous cholesterol synthesis pathway, leading to intracellular accumulation of 7-DHC in SLOS cells (owing to the failure to convert 7-DHC to cholesterol). The effect of 7-DHC storage on free cholesterol transport was assessed by re-addition of LDL to the culture medium for 24 h following an initial 5-day incubation in medium supplemented with LPDS. Under conditions of 7-DHC accumulation in SLOS cells LDL derived free cholesterol was found to accumulate in the late endosomes/lysosomes (LE/Lys) system (confirmed by electron microscopy, Mol Genet Metab 2002; 75(4):325-34). These results indicate that dietary cholesterol supplementation may not be fully utilised by SLOS patients.
There is therefore a need to develop improved treatments for SLOS and related disorders.